diff --git a/src/library/controller.js b/src/library/controller.js
index 9e2807e4a..a2e6ecf9f 100644
--- a/src/library/controller.js
+++ b/src/library/controller.js
@@ -9,6 +9,7 @@
 import { LIBRARY_ENTRIES, LIBRARY_LEDGER } from './catalog.js';
 import { libraryEntryIdOf, libraryLayerId } from './manifest.js';
 import {
+  MAX_SELECTED,
   decodeLibraryParam,
   encodeLibraryParam,
   normalizeLibrarySelection,
@@ -270,14 +271,17 @@ export function createDataLibrary({
     // link, it alone decides what is on, as it does for core layers, so a
     // share with every library entry off switches the stored ones off.
     // Added entries register even while off, so their rows show.
-    for (const id of [...stored.added, ...(linked || [])]) {
+    // Only what the selection keeps registers: past its cap an entry would
+    // be a row the library no longer knows.
+    const next = normalizeLibrarySelection(
+      { added: [...stored.added, ...(linked || [])], on: linked ?? stored.on },
+      switchable,
+    );
+    for (const id of next.added) {
       const entry = byId.get(id);
       if (entry && addable(id)) register(entry);
     }
-    applySelection(
-      { added: [...stored.added, ...(linked || [])], on: linked ?? stored.on },
-      linked ? 'share-restore' : 'local-restore',
-    );
+    applySelection(next, linked ? 'share-restore' : 'local-restore');
   }
 
   /**
@@ -287,6 +291,9 @@ export function createDataLibrary({
    */
   async function add(id) {
     if (destroyed || !addable(id) || pending.has(id)) return false;
+    // A full library refuses the entry before its layer exists.
+    if (!selection.added.includes(id) && selection.added.length >= MAX_SELECTED)
+      return false;
     const entry = byId.get(id);
     const work = (async () => {
       if (!register(entry)) return false;
diff --git a/src/library/controller.test.mjs b/src/library/controller.test.mjs
index 4b75e7b91..4ca490dd6 100644
--- a/src/library/controller.test.mjs
+++ b/src/library/controller.test.mjs
@@ -345,6 +345,64 @@ test('a share opened while an earlier one is still switching on wins', async (t)
   library.destroy();
 });
 
+test('a full library refuses another entry before its layer exists', async (t) => {
+  const many = validateLibraryCatalog(
+    Array.from({ length: 66 }, (_, index) => tiles(`entry-${index}`)),
+  ).entries;
+  const manyLedger = many.map(({ id }, index) => [
+    id,
+    index.toString(36).padStart(2, '0'),
+  ]);
+  const availability = {
+    async getAvailability() {
+      return {
+        server: true,
+        availability: new Map(
+          many.map(({ id }) => [id, { available: true, reason: null }]),
+        ),
+      };
+    },
+    tileTemplate: (entry) => entry.source.url,
+  };
+  const full = JSON.stringify({
+    added: many.slice(0, 64).map(({ id }) => id),
+    on: [],
+  });
+  const manager = sealedManager(t);
+  const library = createDataLibrary({
+    entries: many,
+    ledger: manyLedger,
+    dataManager: manager,
+    services: {},
+    storage: memoryStorage(full),
+    shareLink: fakeShareLink(null),
+    source: availability,
+  });
+  await library.start();
+  assert.equal(manager.layers.size, 64);
+  assert.equal(await library.add('entry-64'), false);
+  assert.equal(manager.layers.size, 64, 'no layer outside the selection');
+  // A share into a full library adds no row the library cannot keep.
+  const shared = sealedManager(t);
+  const opened = createDataLibrary({
+    entries: many,
+    ledger: manyLedger,
+    dataManager: shared,
+    services: {},
+    storage: memoryStorage(full),
+    shareLink: fakeShareLink([manyLedger[65][1]]),
+    source: availability,
+  });
+  await opened.start();
+  assert.equal(shared.layers.has('lib-entry-65'), false);
+  assert.equal(
+    opened.list().filter((row) => row.added).length,
+    shared.layers.size,
+  );
+  library.destroy();
+  opened.destroy();
+});
+
 test('zoom flies to an added layer through the caller, reset removes everything', async (t) => {
   const manager = sealedManager(t);
   const storage = memoryStorage();
diff --git a/src/library/selection.js b/src/library/selection.js
index 513cedf4b..ba565b8c3 100644
--- a/src/library/selection.js
+++ b/src/library/selection.js
@@ -12,7 +12,8 @@ import { LIBRARY_ID_PATTERN } from './manifest.js';
 export const LIBRARY_STORAGE_KEY = 'gev:library:v1';
 export const LIBRARY_SHARE_PARAM = 'lib';
 export const LIBRARY_TOKEN_PATTERN = /^[0-9a-z]{2}$/;
-const MAX_SELECTED = 64;
+/** The most entries one library (and one share link) holds. */
+export const MAX_SELECTED = 64;
 const MAX_PARAM_CHARS = 3 * MAX_SELECTED;
 
 /**